Capillary tube thermal mass flow sensors exploit the fact that heat transfer to a fluid flowing in a laminar tube from the tube walls is a function of mass flow rate of the fluid, the difference between the fluid temperature and the wall temperature, and the specific heat of the fluid. Mass flow controllers employ a variety of mass flow sensor configurations. For example, one type of construction involves a stainless steel flow sensor tube with one, and more typically two or more, resistive elements in thermally conductive contact with the sensor tube. The resistive elements are typically composed of a material having a high temperature coefficient of resistance. Each of the elements can act as a heater, a detector, or both. One or more of the elements is energized with electrical current to supply heat to the fluid stream through the tube. If the heaters are supplied with constant current, the rate of fluid mass flow through the tube can be derived from temperature differences in the elements. Fluid mass flow rates can also be derived by varying the current through the heaters to maintain a constant temperature profile.
Such thermal mass flow sensors may be attached as a part of a mass flow controller, with fluid from the controller's main channel feeding the capillary tube (also referred to herein as the sensor tube). The portion of the main channel to which the inlet and outlet of the sensor tube are attached is often referred to as the “bypass” of the flow sensor. Many applications employ a plurality of mass flow controllers to regulate the supply of fluid through a supply line, and a plurality of the supply lines may be “tapped off” a main fluid supply line. A sudden change in flow to one of the controller's may create pressure fluctuations at the inlet to one or more of the other controllers tapped off the main supply line. Such pressure fluctuations create differences between the flow rate at the inlet and outlet of an affected mass flow controller. Because thermal mass flow sensors measure flow at the inlet of a mass flow controller, but outlet flow from the controller is the critical parameter for process control, such inlet/outlet flow discrepancies can lead to significant process control errors.
In a semiconductor processing application, a process tool may include a plurality of chambers with each chamber having one or more mass flow controllers controlling the flow of gas into the chamber. Each of the mass flow controllers is typically re-calibrated every two weeks. The re-calibration process is described, for example, in U.S. Pat. No. 6,332,348 B1, issued to Yelverton et al. Dec. 25, 2001, which is hereby incorporated by reference. In the course of such an “In Situ” calibration, conventional methods require a technician to connect a mass flow meter in line with each of the mass flow controllers, flow gas through the mass flow meter and mass flow controller, compare the mass flow controller reading to that of the mass flow meter and adjust calibration constants, as necessary. Such painstaking operations can require a great deal of time and, due to labor costs and the unavailability of process tools, with which the mass flow controllers operate, can be very costly.
A mass flow sensor that substantially eliminates sensitivity to pressure variations would therefore be highly desirable. A convenient calibration method and apparatus for mass flow controllers would also be highly desirable. More flexible access to a mass flow controller would also be highly desirable. Apparatus and method for increasing the control performance of a mass flow controller would also be highly desirable.